1. Field of the Invention
The present invention relates to an active retina implant with a multiplicity of pixel elements that convert incident light into electric stimulation signals for cells of the retina to be contacted with stimulation electrodes, each pixel element having at least one image cell that converts incident light into electric signals, and having at least one amplifier whose input is connected to the image cell and whose output is connected to at least one stimulation electrode to which it supplies a stimulation signal, and with an energy supply which provides externally coupled external energy as supply voltage for the image cells and the amplifiers.
2. Related Prior Art
Such a retina implant is disclosed, for example, in DE 197 05 988 A1.
The known retina implant serves the purpose of counteracting a loss of visual faculty based on retina degenerations. The basic idea here is to implant in the region of a patient's degenerated retina a microelectronic stimulation chip that is intended to replace the function of, for example, degenerated photoreceptors. The stimulation chip has a multiplicity of pixel elements that generate electric pulses in the region of the retina as a function of the incident visible light, and in so doing stimulate cells in the retina.
The retina implant can be mounted on the retina as an epiretinal implant, or else it can be inserted in or under the retina as a so-called subretinal implant.
A subretinal implant is disclosed, for example, in EP 0 460 320 A2. With this implant, the impinging ambient light is said to be sufficient for producing the required stimuli for the cells in the retina. Reference is made to this document for the precise placing of a subretinal retina implant.
In DE 197 05 988 A1 mentioned at the outset, a subretinal implant is described that is provided with a photovoltaic layer that is active for nonvisible electromagnetic radiation, the stimulation signals being switched locally by utilizing the voltage generated by the photovoltaic layer. The known implant is based on the idea of using electromagnetic radiation from the nonvisible spectral region, specifically infrared radiation, to provide an external energy for the stimulation chip. The photovoltaic layer acts in this case like a type of amplifier for the signals generated by the incident visible light. As a consequence of this, stimulation signals of adequate intensity can be generated even given weak light conditions in the visible spectral region.
However, the problem arises with the known retina implant of transforming the incident visible light into corresponding electric stimulation signals over a large intensity range that comprises a number of powers of ten in natural light conditions.
Against this background, DE 199 21 399 A1 describes a retina implant having at least one pixel element that acts as reference element, the amplifier forming the difference between the output signals of the reference element and the image cell that detects the local brightness. The aim in this way is to adapt the stimulation signal thus generated to the ambient brightness.
The article by Stelzle et al.: “Electrical Properties of Micro-Photodiode Arrays for Use as Artificial Retina Implant”, Biomedical Micro Devices 3:2, 133-142, 2001 is concerned with the problems of transmitting stimulation signals via the stimulation electrodes to cells of the retina with which contact is to be made. The authors report that the coupling between the stimulation electrode and the tissue is of a capacitive nature such that only transient signals can be used for the stimulation. This capacitive coupling is based on the fact that as a consequence of electrode polarization a capacitance (Helmholtz double layer) is formed in the eye at the interface between electrode and electrolyte. The authors show that for a passive implant, that is to say an implant such as is described in the EP 0 460 320 A2 mentioned at the outset, pulsing of the visible useful light leads to a limit cycle in which a balanced charge transport into the capacitance and out of the latter again results. In order to solve the problems associated with the passive implant, the authors propose to use light pulses with a specific pulse rate. Furthermore, it is said to be desirable to use an external energy supply in order to generate the stimulation current. They also recommend the use of an active current sink in order to reduce the mean electrode polarization. However, they mention that it is very possible that a complete discharge of the electrode capacitance cannot be achieved because of the pulsed excitation.